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Preparation, activity and durability of promoted platinum catalysts for automotive exhaust control
Applted Catalysrs B Envtronmental, 3 (1994) 191-204 Elsevler Science B V , Amstmdam
191
APCAT B60
Preparation, activity and durability of promoted platinum catalysts for automotive exhaust control J.R Gonzalez-Velasco, J. Entrena, J A Gonzalez-Marcos, J I. GutkrezOrtiz and M.A Gutkrez-Ortrz Department of Ckemxal Engtneermng, Faculty of Sciences, Untversulad de1 Pals VaseolEuskal Herrlko Unzbertsrtatea, P 0 Box 644,48080 Bdbao (Spat) (Received 12June 1993)
Abstract The effects of the addltlon of calcla, cena and lanthana to alumma-supported platmum catalysts on the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide automobile emlsslons (three-way catalyst behavlour) were analyzed The actlvlty of the prepared samples was determined wth steady-state, reducmg and oxldlzmg, snnulated feedstreams as well as mth a cycled oxl&zmgreducmg feedstream averaged at the stolchlometnc condltlons which resembled the exhaust am/fuel fluctuations m a closed-loop emlaslon control system Actmty of the catalysts was also analyzed after conducting accelerated thermal and chemical agemg m order to test their durability Under normal operating condltlons of the automobile engme, Pt/AIZOs catalysts promoted by rare-earth oxides are able to achieve high HC, CO and NO convennons The behavlour of the catalysts m the cold start penod was determmed by analysis of light-off temperatures and a comparison was made ~th those correspondmg to some commercial samples and others reported m the hterature The catalysts prepared m this work showed lower hght-off temperatures than those of commemal and reported Pt/Al,O, catalysts but these temperatures were not so low as with Pt-Rh/Al*O( In all cases, the prepared catalysta resulted m a better realstance to accelerated agemg Samples with cena showed the best resistance to accelerated agemg Key words automobile exhaust control, calcla, cena, lanthana, platmum, promoter actmty, way catalysts
three-
INTRODUCTION
Platmum and/or palladmm and rhodmm are the most common active phases in three-way catalysts (TWC ) . New stricter emission control regulations for exhaust gases and the high cost of noble metals have led to mvestigatlons to reduce the cost of these catalysts by improving then activity and durability Correspondence to Dr J R Gonzalez-Velasco, Department of Chemical Engmeenng, Faculty of Sciences, Umvemdad del Pans Vasco/Euskal Hemko Umbertaltatea, P 0 Box 644,48080 Bllbao, Spain Tel ( + 34-4)46477OO X 2433, fax ( + 34-4)4648500
0926-3373/94/$07 SSDI 0926-3373(
00 0 1994 Elsevler Science B V All nghta reserved 93)E0030-4
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Research m thus field includes the optmuzation of the porosity and surface area of the support, the addition of promoters, the reduction of precious metal loadings and an adequate distribution in order to improve the behavlour of the catalyst [ 1 ] The role of platinum in the TWC is to oxidize the CO and hydrocarbons (HC ) , especmlly during the cold start-up of the engme Rh&um is active to reduce mtrogen oxides (NO,) especially m the presence of a high concentration of CO [2] Recent developments in TWC technology may have the potential to reduce the amount of rhodmm needed in TWC formulations as rhodmm is currently used at loadmgs above the average platinum-to-rho&urn ratio m raw ore m most commercial TWC [ 1 ] This has prompted extensive efforts to develop rhodium-free TWC with adequate mtric oxide reduction activity [3-51 Without any ad&tion, the conventional formulation based on transition alumma-supported platinum and/or rhodium catalysts is very efficient m steadystate combtrons constant temperature, flow rate and concentration of the exhaust gases, 1 e , when an enpne 1s operated at the stoichiometrrcally balanced air/fuel (A/F) ratio (stoichiometric pomt ) The real situation is far from bemg so simple and the operating conditions are among the most complex encountered m mdustrml heterogeneous catalysis. The transformation of transition alumina8 to cx-alumma or corundum occurs above 900” C, a temperature which can be occasionally reached in a catalytic converter It is a known expedient to stabihxe the alumma agamst such thermal degradation by the use of materials such as zlrcoma, titarua, alkaline earth metal oxides such as baria, calcla or strontia or, most usually, rare-earth metal oxides, for example, ceria, lanthana and mixtures of two or more rareearth metal oxides [4,6-l 1 ] Durmg operation of a warmed-up TWC in an automobile, the mass A/F ratio cycles about the storchiometric pomt as a result of the characteristics of the A/F control system [ 11,121 In thus sense, the ad&tion of ceria to the catalyst has been reported to increase its activity as well as widen the A/F wmdow of effective operation [ 12-151. An enhancement m the rate of the water-gas shift reaction over ceria under cycled conditions is believed to be largely responsible for the observed beneficial role of cena [ 16,171 Furthermore, oxygen uptake measurements suggest that cerla supphes oxygen to the system m the fuel-rich segment of the exhaust composition cycle and removes excess oxygen from the system in the oxygen-nch segment [13,15,18-201, although this beneficial role under cychng comhtions is somewhat controversial. We have tried to understand the processes that determine the performance of TWC under dynamic conditions so that we could design improved catalysts and A/F control strategies and thereby reduce the cost and complexity of emission control systems. The present study was conducted to probe the effects of the add&ion of calcium, lanthanum and cermm oxides to rhodmm-free alumma-supported platmum catalysts on simultaneous control of hydrocarbon,
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carbon monoxide and nitrogen oxide automobile emissrons Steady-state and cycled laboratory tests have been employed to compare the performance of different catalyst preparations Furthermore, the role of a gaseous environment m thermal agemg and subsequent activity of the catalysts were studied. EXPERIMENTAL
Materuds The startmg alumma (SCS-79) was supplied by Rhone-Poulenc Essentially it was 6-AlsO3 with the following charactenstlcs: BET surface area, 110 m2 g-l, pore volume, 0.58 cm3 g-l, average pore radius, 71 A, predommant pore radms, 50 A,isoelectric point, 6 5 Calcmm, lanthanum and cerium oxides were mcorporated by the conventional mclplent wetness method from an aqueous solution of their correspondmg nitrates, at 40 ’ C and 40 mmHg. Promoter-modified alumma samples were dried at 120” C for 16 h and calcmed m an at 700°C for 4 h Samples with 1,5, 10 and 20 wt -% of promoter were prepared Platinum was incorporated by adsorption from a solution using hexachloroplatmic acid Determination of the adsorption isotherms at 25°C and usmg 40 cm3 of solution per gram of promoter-modified alumma mdlcates that an excess of 90% adsorption was obtamed after 1 h of cantact. The final activation of the precursors was made by calcination at 550’ C m a nitrogen atmosphere for 4 h and subsequent treatment m a H2/N2 (5/95) stream for two additional hours Actrvrty tests Catalytic activity data were obtained by usmg a conventional fixed-bed flow reactor at atmospheric pressure A stamless steel tube with an mner diameter of 12 mm was chosen as the reactor tube Catalyst (3 5 cm3, ca 2 3 g) was placed on ceramic wool at the lower part of the reactor The upper part of the catalyst bed was packed with 10 cm3 of mactive ceramic spheres (2 mm 0 D ) for preheating the feed gas. The furnace temperature was controlled with a maximum variation of 2°C by an automatic temperature controller The gas leaving the reactor was led to a condenser to remove water vapour The remanung components were contmuously analyzed by non dispersive infrared (CO and CO, ) , flame lomzation (HC ) , magnetic susceptibihty (0,)) and chemilummescence (NO,). In order to test the activity of the prepared catalysts on hard conditions we chose methane (considered as the most unreactive alkane) to simulate hydrocarbon m the feedstream and we operated at space velocity m the upper limit of the range normally used, 100 000 h-l. The rate of alkane oxidation increased
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with the molecular weight of the n-alkane from methane to butane [ 211, so that when good results are obtamed under such comhtions, much better OXIdatlon rates wrll be obtained for real automobile exhaust gases, particularly when workmg at the usual space velocrties between 50 000 and 75 000 h- ’ The stoichiometry number, S, used to ldentrfy the redox characterlstlc of the model gas mixtures 1sdefined as
s= 2 [@AI+ [NOI [CO1+4[CH,l
(1)
When SC 10, S= 1.0, and S> 10, the composrtlon of the feedstream is net reducmg, storchrometnc, and net ox&zing, respectively The feed cornpositrons used m the present work were as follows: (1) Reducing feedstream It was composed of 10% COz, 1% CO, 1000 ppm NO, 1000 ppm CHI, 0 35% 02, and a balance of N2 This mixture corresponded to S = 0 57 and simulated a A/F ratro of 14.41 (rich mixture ) (2) Oxldizmg feedstream It consrsted of 10% COz, 1% CO, 1000 ppm NO, 1000 ppm CHI, 0.95% 02, and a balance of Nz. This mixture corresponded to S = 143 and simulated a A/F ratio of 14 86 (lean mixture ) To mvestigate the TWC behavrour of the samples m an environment which resembled the exhaust A/F fluctuations m a closed-loop emission control system we used a similar apparatus to that developed previously by Schlatter et al [ 221 ‘Iwo fast-acting solenoid valves allowed one to cycle between the two above-mentioned feed&reams prepared m two independent gas blendmg systems The prepared catalysts were tested cycling the reducing and ox&zing feedstreams with a frequency of 1 Hz and an amplitude of 0 225 A/F around the mean S = 1.0 and an A/F ratio equal to 14.63 (stolchiometrlc comhtlons ) The space velocity of the above-mentioned reaction gases was kept at 100 000 h- ’ (273 K, 1 atm). Conversron data were measured at temperatures between 250°C and 700” C The hght-off temperature which 1s necessary to obtain a 50% conversion, Tm, was determined from the actlvrty data Agerng tests
Pure alwnma as well as the promoter-modified alummas were calcmed at temperatures between 500 and 1200 oC m order to find out the decrease m BET surface area attributable to the transition to the a-form of alumma and/or smtering of alumina particles. In order to study the durabrlity of the prepared catalysts, they underwent an accelerated agemg consistmg of two consecutive steps: (1) calcmatlon of the samples at 700” C for 14 h m a nitrogen atmosphere, and (2) mamtammg the temperature at 700’ C wrth a subsequent treatment under an oscillatmg stream of CO/N2 (2/98) and 02/Nz (5/95) w&h a frequency of 10T3 Hz for 14 h The behavlour of the aged platinum catalysts with and without promoters was com-
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195
pared wrth that of the fresh catalysts runnmg under the above-mentioned tivrty tests
ac-
RESULTS AND DISCUSSION
Catalyst chzracterzzatwn
Table 1 shows the BET areas correspondmg to the fresh and calcmed alumma under lfferent condrtlons A relative stabrhty of the alumma towards thermal smtermg can only be encountered when dealing wrth dry an m the range 700-900 oC This fact as well as the decomposltlon temperatures of the promoter nitrates to their correspondmg oxldes (determmed by thermogravlmetric analyst Ca(NO,),*4H,O, 7OO”C, Ce(NO,),.4H,O, 3OO”C, La(N0,)s*6H,0, 640°C) allowed us to calcme the promoter-mo&fied alumina at 700°C for 4 h Surface areas of the alummas modified by mcorporatlon of various promoter percentages are shown m Fig 1 Calcla percentages above 5 wt -% gave a slgTABLE 1 BET areas of pure alumma SCS-79 after various thermal treatments BET area, m* g-’
Thermal treatment Gas flow
Temperature, “C
Time, h
Initial sample AU AU Air Ax Air
500 700 900 1000 1200
14 14 14 4 3
0
5
10
15
110 112 107 87 20 10
20
Promoter loading. %
Fig 1 Evolution m the surface area wth the promoter loadmgs for fresh catalysts (0) cermm, ( A ) calcla
( 0 ) lanthana,
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TABLE 2 Charactenstlcs of the prepared catalysts (nommnal Pt = 0 1 wt -% ) Catalysta
0 0 0 0 0 0 0
1PtS lPtSCa1 lPtSCa5 1PtSCel lPtSCe5 lPtSLa1 lPtSLa5
BET surface (m’ g-l) Fresh
Agedb
110 111 97 110 109 116 109
87 85 70 91 86 97 89
Actual Pt wt. -%
Dispersion (%)
0 101 0 122 0 082 0 097 0 085 0 089 0 074
47 34 13 49 53 18 80
“0 1= normal percentage, Pt =preclous metal, S = alumma (SCS-79), I,5 = loadmgs of promoter “Air. 9OO”C, 14 h
Ca, Ce, La =promoters,
n&ant decrease ( > 20% ) of the BET area whilst a much lower decrease was produced by lanthana and cerla Thus, mcorporatlon of 0 1 wt -% platmum was made to the samples with 1% and 5% promoters followmg the procedure mentioned m the experimental section The catalysts were exposed to an at 900” C for 14 h and the surface area was measured after that resulting m the values shown m Table 2 It can be deduced that samples with small percentage (1%) of lanthanum and cerlum show better resdance to thermal degradation of the alumina The actual content of platinum as well as the dlsperslon of the prepared catalysts are shown m the last two columns of Table 2 With Increasing promoter loadmg, the apparent Pt dlsperslons decrease for calcium, mcrease for cerlum and first decrease and then increase for lanthanum The decrease m &sperslon can be ratlonahzed easily, but the Increase m drsperslon 1s lfficult to explain Summers and Ausen [ 231 explained that when Pt has been exposed at high temperature m an and m the presence of large quantities of metal oxide, the product resulting from platmum-promoter interaction can adsorb more than one molecule of the adsorbate gas per platmum site The nature of the mteractlon 1sa function of the particular noble metal, agemg temperature, and gaseous environment TWC behavwur Table 3 shows the light-off temperature, ?!‘a, and the conversion reached at 500” C, Xm, which resulted from the experimental runs mth the prepared fresh and aged catalysts, and using reducmg, ox&zmg and cycled feedstreams, for ehmmatlon of carbon monoxide, mtnc oxide and methane
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197
TABLE 3 T,(“C)
andX,(%)
oftestedcatalysta
Catalyst
co
NO
C&
promotor
Wlthout promotor
Reducing Oxldlzmg Cycled
1% CaO
Reducing Oxldlzmg Cycled
5% CaO
Fkducmg Oxldlzmg Cycled
1% CeO,
Reducing Oxldlzmg Cycled
5% CeOz
Reducing Oxldlzmg Cycled Reducing
Oxldlzmg Cycled Reducing
Oxldumg Cycled
Fresh
Aged
Fresh
Aged
410 62 325 99 7 335 953
460 56 370 996 375 960
475 67
505 47
16 360 73
33 420 73
390 76 340 998 360 989
480 55 390 99 6 400 94 9
445 72
535 38
440 55 300 996 320 94 7
490 52 360 992 385 92 8
400 71 330 999 350 995
435 68 370 99 9 395 98 1
395 71 320 998 330 990
11 410 64
40
435 63
495 53
20 355 78
11 430 59
425 91
485 60
17 370 62
3 425 62
400 71 340 99 9 345 998
395 85
360 94 6
30 360 93
375
425
86 315 999 335 998
75 355 999 375 998
400 70 310 999 320 99 1
Fresh
Aged
10 620 4 550 29
625 4 585 19
630 10 640 6 570 24
4 630 12
620 9 620 9 535 39
5 650 5 600 18
4
13 630 6 545 26
8
25
35 355 76
3 615 7 490 62
19 630 5 515 42
395
465
600
95
78
11 350 63
10 400 55
15 640 7 550 22
8 640 5 565 22
425
410
425
69 345 998 350 98 5
975
87
35 325 78
25 360 71
13 660 6 570 31
10 670 4 590 28
6
JR
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The Pt/Al,O, catalyst, without promoter, worked well m the CO oxldatlon when oxl&zmg and cycled feed&reams were used However, methane OX&Itlon was less satisfactory m an oxygen than m a reducing environment because the simultaneous reactlon CH, + ~NO-D~N, + 2Hz0 + CO, does not occur on alumina-supported platinum m the presence of oxygen m the reactants’ mixtures. Methane 1sfirst oxldlzed and ISonly able to reduce NO when oxygen 1s removed from the feed On the other hand, the oxrdatlon kinetics of alkane hydrocarbon on Pt does not show any mhlbltlon when the partial pressure of the alkane 1s increased because alkanes are not strongly chemlsorbed In the presence of oxygen [ 211 With cycled feedstream, NO conversions around 75% were obtamed at temperatures above 400 ’ C CO, NO and CH4 conversions between 250 and 700°C for this catalyst are represented by thinner lines m Fig 2, for oxl&zmg, reducmg and cycled feed&reams
a
b
0 +
loo-
g
B
60.
P 0”
60 40 t
300
400
500
600
700
Temperature,°C
Fig 2 Conversion vs temperature Thinner lines, unpromoted catalyst, thicker lines, 5 wt -% CeOz promoted catalyst (Triangles) CO oxidation, (circles) NO reduction, (squares) CH, 0x1datlon (a) Oxldlzmg feedstream, (b ) reducing feed&ream, (c ) cycled feed&ream
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Effect of promoter nature and hdang
Table 3 shows that addition of calcium to the catalyst hardly promotes the activity of platinum, although a reduction m the light-off temperatures can be observed under different condltlons. Moreover, the addition of calcium oxide has either no effect or has a negative effect (accordmg to the loading) on the thermal stabilization of the alumina (Table 2 ) Lanthana significantly improves the behavlour of Pt/Al,O, catalyst obtammg higher CO, NO and CH, conversions under reduction conditions. Higher loadings of lanthanum increase the NO conversion with small decrease m the conversion of CO. Under ox&zing comhtions the NO conversion of low lanthanum loadmg was poorer than that of the Pt/A1203 catalyst but became higher for the catalyst with high lanthanum loadmg CO and CHI conversions are mamtamed almost mdependently of the promoter loadmg. By using a cycled feedstream higher CO conversions are obtained m relation to the Pt/Al,O, catalyst but no significant improvement m the reduction of NO and oxlckzatlon of CH, can be noted. Similar trends were observed with the cerium-promoted catalysts Fig 2 shows the CO, NO and CH, conversions for the unpromoted (thm lines) and cermm-promoted (thick lines) catalysts under oxidizing, reducing and cycled conditions The beneficial effect of the addition of ceria to the catalyst can be clearly seen The role of cerium oxide is not only to thermally stabilize the active alumina but also to extend the window of the an/fuel ratio where the catalyst can work to reduce NO, and to oxidize CO and HC simultaneously due to the so-called oxygen storage capacity of cena (e g [ 20,24,25] ). Because of the low redox potential between Ce3+ and Ce4+, CeO, dominate8 m the oxldatlve atmosphere, wlnle under reduction conditions Cez03 become8 predommant 2Ce02&ez03+‘/,0, Thus, especially under a cycled rich-lean composition fluctuation m the feedstream, the cerium oxide8 can either provide oxygen for the oxidation of CO and HC or remove oxygen from the gas phase for the reduction of NO (Fig. 2c) Effect of the fee&ream envrronment
Under oxidizing conditions (Fig. 2a) platmum acts as a very good catalyst at oxldizmg carbon monoxide gnnng an almost total conversion above 350’ C and a hght-off temperature of 325’ C. In this environment, methane 18 oxidized until high conversion but only at very high temperatures resultmg m a hghtoff of 620 ’ C. Nitric oxide conversion 1mcreases up to a maximum temperature of 350°C and decreases after this temperature which corresponds to the absence of carbon monoxide m the environment wbmh prevents the redox reaction NO + CO-t l/zNZ + CO, from takmg place In contrast, under reducmg condltmns (Fig 2b) nitric oxide conversion
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around 60-70% can be obtained but with lower conversions in the oxnhzmg reaction of carbon monoxide and methane than those obtamed under oxl&zmg conditions. Ceria produces a significant decrease m the hght-off temperature for NO conversion as well as a poorer CH, conversion m relation to the promoter-free catalyst under reducing conditions. The cycled rich-lean feed&ream (Fig 2c) produces a very sigmficant lmprovement m the oxldatlon of methane and the reduction of mtric oxide, reducing their hght-off temperatures and mamtannng nearly total conversion of carbon monoxide, especially when ceria ISpromoting the Pt/A1203 catalyst At the normal operating temperature of the automobile engme, 500-550 ’ C, a high conversion of the three pollutants can be achieved. Effect of agerng Fig 3 shows the effect of ageing on the behavlour of the unpromoted, Lapromoted and Ce-promoted catalysts when operating under cycled conditions The light-off temperatures and conversions at 500 ‘C for these catalysts are also shown m Table 3 under reducing, oxidizing and cycled condltlons Fig 3 shows that ageing causes an X-translation of the conversion curves, 1.e the light-off temperatures increase after agemg, however, these increases are smaller with addition of rare-earth oxides to the catalyst (compare Figs 3b and 3c with Fig 3a), especially with CeO, which acts as a good stabilizer against the accelerated ageing improving the durability of the catalyst Around 600” C the conversions of the three pollutants (CO, NO, and CH,) for the aged CeO,-promoted catalysts are almost the same as those of the fresh catalyst NO conversion data for the catalyst with 5 wt.-% CeO, (Table 3) show a surprising decrease in the light-off temperature of the aged catalyst related to the fresh catalyst. This fact can be explained by the shape of the curves between 400 and 600” C which approach one another, as can be seen m Fig. 3 Comparuon wrth reported and commerczal catalysts In order to compare our platinum-based catalysts with the data of other researchers, some papers were selected from the hterature [ 26-281 As can be seen in Table 4 it is not easy to compare the exhaustively reported data with ours because of the different experimental conditions, feedstream flow rates and compositions used. In all cases the space velocity of gasesthrough the catalyst bed is about 20 000-30 000 h-l, 1.e , the contact time between reactants and catalyst is about 4-5 times that of the expenments (100 000 h-’ ), which means advantageouscomhtions for the attainment of a lower hghtoff temperatureand a higher conversion. In general, our catalysts reached sim11aror higher CO and NO conversions although their hght-off temperatures
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201
a 0 200
306
400
500
300
406
506
600
700
660
700
Temperature,°C
3 Converrnon VB temperature Thicker lmes, fresh catalyst, thinner lines, aged catalyst (Tnangles) CO oxldatlon, (cucles) NO reduction, (equaree) CH, ox&&Ion (a) Unpromoted catalyst, (II) LalOB-promoted catalyst, (c ) CezOz-promoted catalyst
Ftg
were lower than those of reported platinum-based catalysts [ 27,28 1, but hgher than those of catalysts with rhodmm reported by Taylor and Sinkevrtch [ 261 We tested two commercial catalysts m order to compare the behaviour of fresh and aged samples throughout the whole range of temperatures under the same experimental condrtlons: Cat A, pelletlzed rhodmm-free, 0.034 wt -% pt, and Cat. B, monolith 0.0165 wt -% Rt and 0 037 wt -% Rh. Rg. 4 shows the CO, NO and CH, conversions for fresh and aged catalysts A and B obtamed m our expenmental setup under the act&y tests mentioned m the expenmental part of the present work Results shown m Fig. 3c can be considered as mtermedrate between those of Figs 4a and 4b, 1e , the O.lPtSCe5 catalyst prepared in thus work clearly shows supenor behavlour to that of commercial rhodmm-free catalysts both in the cold start period and under normal operatmg conditrons of the automobile engme Nevertheless, our best rho-
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TABLE 4 Charactenstlcs of some catalysts reported m literature Llghtoff temperature ( oC )
Catalyst metal loadmgs (wt -%)
Authors
co
NO
Taylor and Smkevltch [ 261
Pt, 0 0682 Pd, 0 0313 Rh, 0 007 CeOz, 2 68
290-b
305”b
D’AnJlo al WI
Pt,Ol
360b
NA
Cho [28]
Pt,Ol
450”d 300’b
420’ d
Thu work
Pt, 0 104 CeO,, 5 0
330
360
et
Space velocity, (h-l)
20 000
NA 30 000 loo 000
‘Reactor bed temperature bLaboratory test “Reactor mlet temperature dEngme-dynamometer test
a 300
400
500
600
500
600
b 400
700
Temperature,
OC
Fig 4 Conversion vs temperature for commercial catalysts Thicker hnes, fresh catalyst, thmner hnes, aged catalyst (Tnangles) CO oxldatlon, (circles) NO reduction, (squares) CH, oxldatlon (a) Pt/A1203 catalyst, (b) PtRh/AlzOB catalyst
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dmm-free catalyst does not behave as well as the commercial Rh/Pt catalyst in the cold start period, although similar or even better behavlour can be observed at normal operation of the automotive engine, 1 e , conversion obtained at 550°C CONCLUSIONS
This paper has described the effects of the addition of calcmm, lanthanum and cermm oxides to rhodium-free 0 1 wt.-% Pt/A1203 catalysts on slmultaneous control of methane, carbon monoxide and nitrogen oxide automobile emissions Ad&tion of calcmm oxide produced the most sqnificant decrease of the surface area of the alumma without enhancement m the activity of the catalyst Incorporation of 1 wt -% rare-earth oxides to the catalysts produced good stabilization of the alumina towards thermal smtering up to 909 oC, higher loadmg (5 wt.-% ) had almost no effect on the thermal stab&y of alunnna These promoted catalysts showed an improved three-way behavlour allowmg high CO, CH, and NO conversions snnultaneously, especmlly under simulated cycled conditions between reducing and oxidizing feedstreams which resemble the exhaust A/F fluctuations in a close-loop emission control system The cermmpromoted catalysts produced the best results due to their so-called oxygen storage capacity which provided oxygen for the oxidation of CO and HC or removed oxygen for the reductron of NO under cycled rich-lean feedstream compositions On the other hand, the prepared rare-earth oxide-promoted catalysts showed good res&ance to the accelerated agemg to which they were submitted Under normal operating conditions of the automobile engine, 55O”C, high HC, CO and NO conversions can be achieved, and the light-off temperatures are mamtamed below 350’ C for CO and NO The prepared rhodium-free 0.1 wt -% Pt/A1203 catalysts showed better behavlour from the point of view of both activity at normal engine operation as well as cold start penod (low light-off temperature) compared with that achieved by commercial Pt/Al,O, samples and others reported m the literature [ 26-281 Nevertheless, it has not been possible to achieve as low light-off temperatures as those obtained with commercial PtRh/AlzOS catalysts However, the catalysts presented m this work had a better resistance to accelerated agemg The fore-mentioned allows us to conclude that rhodmm loading m rare-earthpromoted catalysts can be drastically reduced without any detriment to almultaneous CO, HC, and NO conversions achieved during normal operating conditions of the automobile engine, and may further lead to a slight mcrease in light-off temperature, and even an improvement m the resistance to accelerated agemg
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ACKNOWLEDGEMENT
The authors would like to recognize the financial support received from the Umversxiad de1 Pals Vasco (Projects E147/90 and E150/91) and from the Goblerno Vasco ( ProJect GV89A4 )
REFERENCES
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K C Taylor, Automobile Catalytic Converters, Sprmg-Verleg, Berhn (1984) J C Summers and L L Hegedus, J Catal, 51 (1978) 18s K M Adams and H S Gandhi, Ind Eng Chem Prod Res Dev ,22 (1983) 207 H Murakl, H ShmJoh and Y FuJltanl, Appl Catal ,22 (1986) 325 H Murakl, K Yokota and Y Fu~dnl, Appl Catal ,48 (1989) 93 P Nortier and M Soustelle, m A Crucq and A Frennet (Editors), Catalysis and Automotive Pollution Control, (Studies Surface Science and Catalysis, Vol 30)) Elsevler, Amsterdam, 1987,~ 275 V A Drozdov, P G Tsyrul’mkov, A N Pestryakov, A A Davydov and V V Popovsku, Kmet Katal ,29 (1988) 484 B Hamson, A F Dlwell and C Hallet, Platm Met Rev ,32 (1988) 73 C Z Wan (to Engelhard Corporation), European Patent 0 303 495 Al (July, 1989) H Murakaml, K Yamsgata and K Ihars (to Mazda Motor Corporation ) , European Patent 0 407 915 A2 (March, 1991) B Begum, E Garbowsh and M Pnmet, Appl Catal ,75 (1991) 119 L L Hegedus, J C Summers, J C S&latter and K Baron, J Catal ,56 (1979) 321 R K Hen, Ind Eng Chem Prod Res Dev ,20 (1981) 451 G Kun, Ind Eng Chem Prod Res Dev ,21 (1982) 267 H C Yao and Y F Yu Yao, J Catal, 86 (1984) 254 J C S&latter and P J Mitchell, Ind Eng Chem Prod Res Dev ,19 (1980) 288 R H Herz and J A Sell, J Catal, 94 (1985) 166 I S Metcalfe and S Sundaresan, Chem Eng SCI,41 (1986) 1109 B Engler, E Koberstem and P Schubert, Appl Catal ,48 (1989) 71 T M&l, T Ogawa, M Haneda, N Kakuta, A Lleno, S Tatelshl, S Mataura and M Sat.o, J Phys Chem ,94 (1990) 6464 J T Kummer, J Phys Chem ,90 (1986) 4747 J C S&latter, R M Smkevltch and P J Mitchell, Ind Eng Chem Prod Res Dev ,19(1980) 288 J C Summers and S A Ausen, J Catal, 58 (1979) 131 P Loof, B Kasemo and K -E Keck, J Catal ,118 (1989) 339 B Engler, E Koberstem and P Schubert, Appl Catal ,48 (1989) 71 K C Taylor and R M Smkevltch, Ind Eng Chem Prod Res Dev ,22 ( 1983 ) 45 M J D’Amello, D R Monroe, C J Carr and M H Krueger, J Catal , 109 (1988) 407 B K Cho, Ind Eng Chem Res ,27 (1988) 30
191
APCAT B60
Preparation, activity and durability of promoted platinum catalysts for automotive exhaust control J.R Gonzalez-Velasco, J. Entrena, J A Gonzalez-Marcos, J I. GutkrezOrtiz and M.A Gutkrez-Ortrz Department of Ckemxal Engtneermng, Faculty of Sciences, Untversulad de1 Pals VaseolEuskal Herrlko Unzbertsrtatea, P 0 Box 644,48080 Bdbao (Spat) (Received 12June 1993)
Abstract The effects of the addltlon of calcla, cena and lanthana to alumma-supported platmum catalysts on the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide automobile emlsslons (three-way catalyst behavlour) were analyzed The actlvlty of the prepared samples was determined wth steady-state, reducmg and oxldlzmg, snnulated feedstreams as well as mth a cycled oxl&zmgreducmg feedstream averaged at the stolchlometnc condltlons which resembled the exhaust am/fuel fluctuations m a closed-loop emlaslon control system Actmty of the catalysts was also analyzed after conducting accelerated thermal and chemical agemg m order to test their durability Under normal operating condltlons of the automobile engme, Pt/AIZOs catalysts promoted by rare-earth oxides are able to achieve high HC, CO and NO convennons The behavlour of the catalysts m the cold start penod was determmed by analysis of light-off temperatures and a comparison was made ~th those correspondmg to some commercial samples and others reported m the hterature The catalysts prepared m this work showed lower hght-off temperatures than those of commemal and reported Pt/Al,O, catalysts but these temperatures were not so low as with Pt-Rh/Al*O( In all cases, the prepared catalysta resulted m a better realstance to accelerated agemg Samples with cena showed the best resistance to accelerated agemg Key words automobile exhaust control, calcla, cena, lanthana, platmum, promoter actmty, way catalysts
three-
INTRODUCTION
Platmum and/or palladmm and rhodmm are the most common active phases in three-way catalysts (TWC ) . New stricter emission control regulations for exhaust gases and the high cost of noble metals have led to mvestigatlons to reduce the cost of these catalysts by improving then activity and durability Correspondence to Dr J R Gonzalez-Velasco, Department of Chemical Engmeenng, Faculty of Sciences, Umvemdad del Pans Vasco/Euskal Hemko Umbertaltatea, P 0 Box 644,48080 Bllbao, Spain Tel ( + 34-4)46477OO X 2433, fax ( + 34-4)4648500
0926-3373/94/$07 SSDI 0926-3373(
00 0 1994 Elsevler Science B V All nghta reserved 93)E0030-4
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Research m thus field includes the optmuzation of the porosity and surface area of the support, the addition of promoters, the reduction of precious metal loadings and an adequate distribution in order to improve the behavlour of the catalyst [ 1 ] The role of platinum in the TWC is to oxidize the CO and hydrocarbons (HC ) , especmlly during the cold start-up of the engme Rh&um is active to reduce mtrogen oxides (NO,) especially m the presence of a high concentration of CO [2] Recent developments in TWC technology may have the potential to reduce the amount of rhodmm needed in TWC formulations as rhodmm is currently used at loadmgs above the average platinum-to-rho&urn ratio m raw ore m most commercial TWC [ 1 ] This has prompted extensive efforts to develop rhodium-free TWC with adequate mtric oxide reduction activity [3-51 Without any ad&tion, the conventional formulation based on transition alumma-supported platinum and/or rhodium catalysts is very efficient m steadystate combtrons constant temperature, flow rate and concentration of the exhaust gases, 1 e , when an enpne 1s operated at the stoichiometrrcally balanced air/fuel (A/F) ratio (stoichiometric pomt ) The real situation is far from bemg so simple and the operating conditions are among the most complex encountered m mdustrml heterogeneous catalysis. The transformation of transition alumina8 to cx-alumma or corundum occurs above 900” C, a temperature which can be occasionally reached in a catalytic converter It is a known expedient to stabihxe the alumma agamst such thermal degradation by the use of materials such as zlrcoma, titarua, alkaline earth metal oxides such as baria, calcla or strontia or, most usually, rare-earth metal oxides, for example, ceria, lanthana and mixtures of two or more rareearth metal oxides [4,6-l 1 ] Durmg operation of a warmed-up TWC in an automobile, the mass A/F ratio cycles about the storchiometric pomt as a result of the characteristics of the A/F control system [ 11,121 In thus sense, the ad&tion of ceria to the catalyst has been reported to increase its activity as well as widen the A/F wmdow of effective operation [ 12-151. An enhancement m the rate of the water-gas shift reaction over ceria under cycled conditions is believed to be largely responsible for the observed beneficial role of cena [ 16,171 Furthermore, oxygen uptake measurements suggest that cerla supphes oxygen to the system m the fuel-rich segment of the exhaust composition cycle and removes excess oxygen from the system in the oxygen-nch segment [13,15,18-201, although this beneficial role under cychng comhtions is somewhat controversial. We have tried to understand the processes that determine the performance of TWC under dynamic conditions so that we could design improved catalysts and A/F control strategies and thereby reduce the cost and complexity of emission control systems. The present study was conducted to probe the effects of the add&ion of calcium, lanthanum and cermm oxides to rhodmm-free alumma-supported platmum catalysts on simultaneous control of hydrocarbon,
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193
carbon monoxide and nitrogen oxide automobile emissrons Steady-state and cycled laboratory tests have been employed to compare the performance of different catalyst preparations Furthermore, the role of a gaseous environment m thermal agemg and subsequent activity of the catalysts were studied. EXPERIMENTAL
Materuds The startmg alumma (SCS-79) was supplied by Rhone-Poulenc Essentially it was 6-AlsO3 with the following charactenstlcs: BET surface area, 110 m2 g-l, pore volume, 0.58 cm3 g-l, average pore radius, 71 A, predommant pore radms, 50 A,isoelectric point, 6 5 Calcmm, lanthanum and cerium oxides were mcorporated by the conventional mclplent wetness method from an aqueous solution of their correspondmg nitrates, at 40 ’ C and 40 mmHg. Promoter-modified alumma samples were dried at 120” C for 16 h and calcmed m an at 700°C for 4 h Samples with 1,5, 10 and 20 wt -% of promoter were prepared Platinum was incorporated by adsorption from a solution using hexachloroplatmic acid Determination of the adsorption isotherms at 25°C and usmg 40 cm3 of solution per gram of promoter-modified alumma mdlcates that an excess of 90% adsorption was obtamed after 1 h of cantact. The final activation of the precursors was made by calcination at 550’ C m a nitrogen atmosphere for 4 h and subsequent treatment m a H2/N2 (5/95) stream for two additional hours Actrvrty tests Catalytic activity data were obtained by usmg a conventional fixed-bed flow reactor at atmospheric pressure A stamless steel tube with an mner diameter of 12 mm was chosen as the reactor tube Catalyst (3 5 cm3, ca 2 3 g) was placed on ceramic wool at the lower part of the reactor The upper part of the catalyst bed was packed with 10 cm3 of mactive ceramic spheres (2 mm 0 D ) for preheating the feed gas. The furnace temperature was controlled with a maximum variation of 2°C by an automatic temperature controller The gas leaving the reactor was led to a condenser to remove water vapour The remanung components were contmuously analyzed by non dispersive infrared (CO and CO, ) , flame lomzation (HC ) , magnetic susceptibihty (0,)) and chemilummescence (NO,). In order to test the activity of the prepared catalysts on hard conditions we chose methane (considered as the most unreactive alkane) to simulate hydrocarbon m the feedstream and we operated at space velocity m the upper limit of the range normally used, 100 000 h-l. The rate of alkane oxidation increased
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with the molecular weight of the n-alkane from methane to butane [ 211, so that when good results are obtamed under such comhtions, much better OXIdatlon rates wrll be obtained for real automobile exhaust gases, particularly when workmg at the usual space velocrties between 50 000 and 75 000 h- ’ The stoichiometry number, S, used to ldentrfy the redox characterlstlc of the model gas mixtures 1sdefined as
s= 2 [@AI+ [NOI [CO1+4[CH,l
(1)
When SC 10, S= 1.0, and S> 10, the composrtlon of the feedstream is net reducmg, storchrometnc, and net ox&zing, respectively The feed cornpositrons used m the present work were as follows: (1) Reducing feedstream It was composed of 10% COz, 1% CO, 1000 ppm NO, 1000 ppm CHI, 0 35% 02, and a balance of N2 This mixture corresponded to S = 0 57 and simulated a A/F ratro of 14.41 (rich mixture ) (2) Oxldizmg feedstream It consrsted of 10% COz, 1% CO, 1000 ppm NO, 1000 ppm CHI, 0.95% 02, and a balance of Nz. This mixture corresponded to S = 143 and simulated a A/F ratio of 14 86 (lean mixture ) To mvestigate the TWC behavrour of the samples m an environment which resembled the exhaust A/F fluctuations m a closed-loop emission control system we used a similar apparatus to that developed previously by Schlatter et al [ 221 ‘Iwo fast-acting solenoid valves allowed one to cycle between the two above-mentioned feed&reams prepared m two independent gas blendmg systems The prepared catalysts were tested cycling the reducing and ox&zing feedstreams with a frequency of 1 Hz and an amplitude of 0 225 A/F around the mean S = 1.0 and an A/F ratio equal to 14.63 (stolchiometrlc comhtlons ) The space velocity of the above-mentioned reaction gases was kept at 100 000 h- ’ (273 K, 1 atm). Conversron data were measured at temperatures between 250°C and 700” C The hght-off temperature which 1s necessary to obtain a 50% conversion, Tm, was determined from the actlvrty data Agerng tests
Pure alwnma as well as the promoter-modified alummas were calcmed at temperatures between 500 and 1200 oC m order to find out the decrease m BET surface area attributable to the transition to the a-form of alumma and/or smtering of alumina particles. In order to study the durabrlity of the prepared catalysts, they underwent an accelerated agemg consistmg of two consecutive steps: (1) calcmatlon of the samples at 700” C for 14 h m a nitrogen atmosphere, and (2) mamtammg the temperature at 700’ C wrth a subsequent treatment under an oscillatmg stream of CO/N2 (2/98) and 02/Nz (5/95) w&h a frequency of 10T3 Hz for 14 h The behavlour of the aged platinum catalysts with and without promoters was com-
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195
pared wrth that of the fresh catalysts runnmg under the above-mentioned tivrty tests
ac-
RESULTS AND DISCUSSION
Catalyst chzracterzzatwn
Table 1 shows the BET areas correspondmg to the fresh and calcmed alumma under lfferent condrtlons A relative stabrhty of the alumma towards thermal smtermg can only be encountered when dealing wrth dry an m the range 700-900 oC This fact as well as the decomposltlon temperatures of the promoter nitrates to their correspondmg oxldes (determmed by thermogravlmetric analyst Ca(NO,),*4H,O, 7OO”C, Ce(NO,),.4H,O, 3OO”C, La(N0,)s*6H,0, 640°C) allowed us to calcme the promoter-mo&fied alumina at 700°C for 4 h Surface areas of the alummas modified by mcorporatlon of various promoter percentages are shown m Fig 1 Calcla percentages above 5 wt -% gave a slgTABLE 1 BET areas of pure alumma SCS-79 after various thermal treatments BET area, m* g-’
Thermal treatment Gas flow
Temperature, “C
Time, h
Initial sample AU AU Air Ax Air
500 700 900 1000 1200
14 14 14 4 3
0
5
10
15
110 112 107 87 20 10
20
Promoter loading. %
Fig 1 Evolution m the surface area wth the promoter loadmgs for fresh catalysts (0) cermm, ( A ) calcla
( 0 ) lanthana,
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TABLE 2 Charactenstlcs of the prepared catalysts (nommnal Pt = 0 1 wt -% ) Catalysta
0 0 0 0 0 0 0
1PtS lPtSCa1 lPtSCa5 1PtSCel lPtSCe5 lPtSLa1 lPtSLa5
BET surface (m’ g-l) Fresh
Agedb
110 111 97 110 109 116 109
87 85 70 91 86 97 89
Actual Pt wt. -%
Dispersion (%)
0 101 0 122 0 082 0 097 0 085 0 089 0 074
47 34 13 49 53 18 80
“0 1= normal percentage, Pt =preclous metal, S = alumma (SCS-79), I,5 = loadmgs of promoter “Air. 9OO”C, 14 h
Ca, Ce, La =promoters,
n&ant decrease ( > 20% ) of the BET area whilst a much lower decrease was produced by lanthana and cerla Thus, mcorporatlon of 0 1 wt -% platmum was made to the samples with 1% and 5% promoters followmg the procedure mentioned m the experimental section The catalysts were exposed to an at 900” C for 14 h and the surface area was measured after that resulting m the values shown m Table 2 It can be deduced that samples with small percentage (1%) of lanthanum and cerlum show better resdance to thermal degradation of the alumina The actual content of platinum as well as the dlsperslon of the prepared catalysts are shown m the last two columns of Table 2 With Increasing promoter loadmg, the apparent Pt dlsperslons decrease for calcium, mcrease for cerlum and first decrease and then increase for lanthanum The decrease m &sperslon can be ratlonahzed easily, but the Increase m drsperslon 1s lfficult to explain Summers and Ausen [ 231 explained that when Pt has been exposed at high temperature m an and m the presence of large quantities of metal oxide, the product resulting from platmum-promoter interaction can adsorb more than one molecule of the adsorbate gas per platmum site The nature of the mteractlon 1sa function of the particular noble metal, agemg temperature, and gaseous environment TWC behavwur Table 3 shows the light-off temperature, ?!‘a, and the conversion reached at 500” C, Xm, which resulted from the experimental runs mth the prepared fresh and aged catalysts, and using reducmg, ox&zmg and cycled feedstreams, for ehmmatlon of carbon monoxide, mtnc oxide and methane
J R Gonzdez-Vehsco et al / Appl Catal B 3 (1994) 191-204
197
TABLE 3 T,(“C)
andX,(%)
oftestedcatalysta
Catalyst
co
NO
C&
promotor
Wlthout promotor
Reducing Oxldlzmg Cycled
1% CaO
Reducing Oxldlzmg Cycled
5% CaO
Fkducmg Oxldlzmg Cycled
1% CeO,
Reducing Oxldlzmg Cycled
5% CeOz
Reducing Oxldlzmg Cycled Reducing
Oxldlzmg Cycled Reducing
Oxldumg Cycled
Fresh
Aged
Fresh
Aged
410 62 325 99 7 335 953
460 56 370 996 375 960
475 67
505 47
16 360 73
33 420 73
390 76 340 998 360 989
480 55 390 99 6 400 94 9
445 72
535 38
440 55 300 996 320 94 7
490 52 360 992 385 92 8
400 71 330 999 350 995
435 68 370 99 9 395 98 1
395 71 320 998 330 990
11 410 64
40
435 63
495 53
20 355 78
11 430 59
425 91
485 60
17 370 62
3 425 62
400 71 340 99 9 345 998
395 85
360 94 6
30 360 93
375
425
86 315 999 335 998
75 355 999 375 998
400 70 310 999 320 99 1
Fresh
Aged
10 620 4 550 29
625 4 585 19
630 10 640 6 570 24
4 630 12
620 9 620 9 535 39
5 650 5 600 18
4
13 630 6 545 26
8
25
35 355 76
3 615 7 490 62
19 630 5 515 42
395
465
600
95
78
11 350 63
10 400 55
15 640 7 550 22
8 640 5 565 22
425
410
425
69 345 998 350 98 5
975
87
35 325 78
25 360 71
13 660 6 570 31
10 670 4 590 28
6
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The Pt/Al,O, catalyst, without promoter, worked well m the CO oxldatlon when oxl&zmg and cycled feed&reams were used However, methane OX&Itlon was less satisfactory m an oxygen than m a reducing environment because the simultaneous reactlon CH, + ~NO-D~N, + 2Hz0 + CO, does not occur on alumina-supported platinum m the presence of oxygen m the reactants’ mixtures. Methane 1sfirst oxldlzed and ISonly able to reduce NO when oxygen 1s removed from the feed On the other hand, the oxrdatlon kinetics of alkane hydrocarbon on Pt does not show any mhlbltlon when the partial pressure of the alkane 1s increased because alkanes are not strongly chemlsorbed In the presence of oxygen [ 211 With cycled feedstream, NO conversions around 75% were obtamed at temperatures above 400 ’ C CO, NO and CH4 conversions between 250 and 700°C for this catalyst are represented by thinner lines m Fig 2, for oxl&zmg, reducmg and cycled feed&reams
a
b
0 +
loo-
g
B
60.
P 0”
60 40 t
300
400
500
600
700
Temperature,°C
Fig 2 Conversion vs temperature Thinner lines, unpromoted catalyst, thicker lines, 5 wt -% CeOz promoted catalyst (Triangles) CO oxidation, (circles) NO reduction, (squares) CH, 0x1datlon (a) Oxldlzmg feedstream, (b ) reducing feed&ream, (c ) cycled feed&ream
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Effect of promoter nature and hdang
Table 3 shows that addition of calcium to the catalyst hardly promotes the activity of platinum, although a reduction m the light-off temperatures can be observed under different condltlons. Moreover, the addition of calcium oxide has either no effect or has a negative effect (accordmg to the loading) on the thermal stabilization of the alumina (Table 2 ) Lanthana significantly improves the behavlour of Pt/Al,O, catalyst obtammg higher CO, NO and CH, conversions under reduction conditions. Higher loadings of lanthanum increase the NO conversion with small decrease m the conversion of CO. Under ox&zing comhtions the NO conversion of low lanthanum loadmg was poorer than that of the Pt/A1203 catalyst but became higher for the catalyst with high lanthanum loadmg CO and CHI conversions are mamtamed almost mdependently of the promoter loadmg. By using a cycled feedstream higher CO conversions are obtained m relation to the Pt/Al,O, catalyst but no significant improvement m the reduction of NO and oxlckzatlon of CH, can be noted. Similar trends were observed with the cerium-promoted catalysts Fig 2 shows the CO, NO and CH, conversions for the unpromoted (thm lines) and cermm-promoted (thick lines) catalysts under oxidizing, reducing and cycled conditions The beneficial effect of the addition of ceria to the catalyst can be clearly seen The role of cerium oxide is not only to thermally stabilize the active alumina but also to extend the window of the an/fuel ratio where the catalyst can work to reduce NO, and to oxidize CO and HC simultaneously due to the so-called oxygen storage capacity of cena (e g [ 20,24,25] ). Because of the low redox potential between Ce3+ and Ce4+, CeO, dominate8 m the oxldatlve atmosphere, wlnle under reduction conditions Cez03 become8 predommant 2Ce02&ez03+‘/,0, Thus, especially under a cycled rich-lean composition fluctuation m the feedstream, the cerium oxide8 can either provide oxygen for the oxidation of CO and HC or remove oxygen from the gas phase for the reduction of NO (Fig. 2c) Effect of the fee&ream envrronment
Under oxidizing conditions (Fig. 2a) platmum acts as a very good catalyst at oxldizmg carbon monoxide gnnng an almost total conversion above 350’ C and a hght-off temperature of 325’ C. In this environment, methane 18 oxidized until high conversion but only at very high temperatures resultmg m a hghtoff of 620 ’ C. Nitric oxide conversion 1mcreases up to a maximum temperature of 350°C and decreases after this temperature which corresponds to the absence of carbon monoxide m the environment wbmh prevents the redox reaction NO + CO-t l/zNZ + CO, from takmg place In contrast, under reducmg condltmns (Fig 2b) nitric oxide conversion
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around 60-70% can be obtained but with lower conversions in the oxnhzmg reaction of carbon monoxide and methane than those obtamed under oxl&zmg conditions. Ceria produces a significant decrease m the hght-off temperature for NO conversion as well as a poorer CH, conversion m relation to the promoter-free catalyst under reducing conditions. The cycled rich-lean feed&ream (Fig 2c) produces a very sigmficant lmprovement m the oxldatlon of methane and the reduction of mtric oxide, reducing their hght-off temperatures and mamtannng nearly total conversion of carbon monoxide, especially when ceria ISpromoting the Pt/A1203 catalyst At the normal operating temperature of the automobile engme, 500-550 ’ C, a high conversion of the three pollutants can be achieved. Effect of agerng Fig 3 shows the effect of ageing on the behavlour of the unpromoted, Lapromoted and Ce-promoted catalysts when operating under cycled conditions The light-off temperatures and conversions at 500 ‘C for these catalysts are also shown m Table 3 under reducing, oxidizing and cycled condltlons Fig 3 shows that ageing causes an X-translation of the conversion curves, 1.e the light-off temperatures increase after agemg, however, these increases are smaller with addition of rare-earth oxides to the catalyst (compare Figs 3b and 3c with Fig 3a), especially with CeO, which acts as a good stabilizer against the accelerated ageing improving the durability of the catalyst Around 600” C the conversions of the three pollutants (CO, NO, and CH,) for the aged CeO,-promoted catalysts are almost the same as those of the fresh catalyst NO conversion data for the catalyst with 5 wt.-% CeO, (Table 3) show a surprising decrease in the light-off temperature of the aged catalyst related to the fresh catalyst. This fact can be explained by the shape of the curves between 400 and 600” C which approach one another, as can be seen m Fig. 3 Comparuon wrth reported and commerczal catalysts In order to compare our platinum-based catalysts with the data of other researchers, some papers were selected from the hterature [ 26-281 As can be seen in Table 4 it is not easy to compare the exhaustively reported data with ours because of the different experimental conditions, feedstream flow rates and compositions used. In all cases the space velocity of gasesthrough the catalyst bed is about 20 000-30 000 h-l, 1.e , the contact time between reactants and catalyst is about 4-5 times that of the expenments (100 000 h-’ ), which means advantageouscomhtions for the attainment of a lower hghtoff temperatureand a higher conversion. In general, our catalysts reached sim11aror higher CO and NO conversions although their hght-off temperatures
JR Gonzrilez-Vektsco et al / Appl Catal B 3 (1994) 191-204
201
a 0 200
306
400
500
300
406
506
600
700
660
700
Temperature,°C
3 Converrnon VB temperature Thicker lmes, fresh catalyst, thinner lines, aged catalyst (Tnangles) CO oxldatlon, (cucles) NO reduction, (equaree) CH, ox&&Ion (a) Unpromoted catalyst, (II) LalOB-promoted catalyst, (c ) CezOz-promoted catalyst
Ftg
were lower than those of reported platinum-based catalysts [ 27,28 1, but hgher than those of catalysts with rhodmm reported by Taylor and Sinkevrtch [ 261 We tested two commercial catalysts m order to compare the behaviour of fresh and aged samples throughout the whole range of temperatures under the same experimental condrtlons: Cat A, pelletlzed rhodmm-free, 0.034 wt -% pt, and Cat. B, monolith 0.0165 wt -% Rt and 0 037 wt -% Rh. Rg. 4 shows the CO, NO and CH, conversions for fresh and aged catalysts A and B obtamed m our expenmental setup under the act&y tests mentioned m the expenmental part of the present work Results shown m Fig. 3c can be considered as mtermedrate between those of Figs 4a and 4b, 1e , the O.lPtSCe5 catalyst prepared in thus work clearly shows supenor behavlour to that of commercial rhodmm-free catalysts both in the cold start period and under normal operatmg conditrons of the automobile engme Nevertheless, our best rho-
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TABLE 4 Charactenstlcs of some catalysts reported m literature Llghtoff temperature ( oC )
Catalyst metal loadmgs (wt -%)
Authors
co
NO
Taylor and Smkevltch [ 261
Pt, 0 0682 Pd, 0 0313 Rh, 0 007 CeOz, 2 68
290-b
305”b
D’AnJlo al WI
Pt,Ol
360b
NA
Cho [28]
Pt,Ol
450”d 300’b
420’ d
Thu work
Pt, 0 104 CeO,, 5 0
330
360
et
Space velocity, (h-l)
20 000
NA 30 000 loo 000
‘Reactor bed temperature bLaboratory test “Reactor mlet temperature dEngme-dynamometer test
a 300
400
500
600
500
600
b 400
700
Temperature,
OC
Fig 4 Conversion vs temperature for commercial catalysts Thicker hnes, fresh catalyst, thmner hnes, aged catalyst (Tnangles) CO oxldatlon, (circles) NO reduction, (squares) CH, oxldatlon (a) Pt/A1203 catalyst, (b) PtRh/AlzOB catalyst
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dmm-free catalyst does not behave as well as the commercial Rh/Pt catalyst in the cold start period, although similar or even better behavlour can be observed at normal operation of the automotive engine, 1 e , conversion obtained at 550°C CONCLUSIONS
This paper has described the effects of the addition of calcmm, lanthanum and cermm oxides to rhodium-free 0 1 wt.-% Pt/A1203 catalysts on slmultaneous control of methane, carbon monoxide and nitrogen oxide automobile emissions Ad&tion of calcmm oxide produced the most sqnificant decrease of the surface area of the alumma without enhancement m the activity of the catalyst Incorporation of 1 wt -% rare-earth oxides to the catalysts produced good stabilization of the alumina towards thermal smtering up to 909 oC, higher loadmg (5 wt.-% ) had almost no effect on the thermal stab&y of alunnna These promoted catalysts showed an improved three-way behavlour allowmg high CO, CH, and NO conversions snnultaneously, especmlly under simulated cycled conditions between reducing and oxidizing feedstreams which resemble the exhaust A/F fluctuations in a close-loop emission control system The cermmpromoted catalysts produced the best results due to their so-called oxygen storage capacity which provided oxygen for the oxidation of CO and HC or removed oxygen for the reductron of NO under cycled rich-lean feedstream compositions On the other hand, the prepared rare-earth oxide-promoted catalysts showed good res&ance to the accelerated agemg to which they were submitted Under normal operating conditions of the automobile engine, 55O”C, high HC, CO and NO conversions can be achieved, and the light-off temperatures are mamtamed below 350’ C for CO and NO The prepared rhodium-free 0.1 wt -% Pt/A1203 catalysts showed better behavlour from the point of view of both activity at normal engine operation as well as cold start penod (low light-off temperature) compared with that achieved by commercial Pt/Al,O, samples and others reported m the literature [ 26-281 Nevertheless, it has not been possible to achieve as low light-off temperatures as those obtained with commercial PtRh/AlzOS catalysts However, the catalysts presented m this work had a better resistance to accelerated agemg The fore-mentioned allows us to conclude that rhodmm loading m rare-earthpromoted catalysts can be drastically reduced without any detriment to almultaneous CO, HC, and NO conversions achieved during normal operating conditions of the automobile engine, and may further lead to a slight mcrease in light-off temperature, and even an improvement m the resistance to accelerated agemg
204
JR
Gonzcilez-Velasco
et al / Appl
Catal B 3 (1994) 191-204
ACKNOWLEDGEMENT
The authors would like to recognize the financial support received from the Umversxiad de1 Pals Vasco (Projects E147/90 and E150/91) and from the Goblerno Vasco ( ProJect GV89A4 )
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